In an optical disk system, an objective lens generally focuses a laser beam emitted from a semiconductor laser onto an optical disk. Since information is spirally recorded on the optical disk, the laser beam scanning the information undergoes modulation. When recording information, the laser beam intensity is modulated along a spiral recording track formed on the optical disk. Since the optical disk is decentered, the position of the above-described spiral structure changes with respect to an optical pickup. To follow this change, the objective lens of the optical pickup is held by an actuator, and the actuator is shifted radial direction of relative to the disk. This allows the laser beam to keep scanning the spiral structure.
When the objective lens is shifted as described above, an object point for the objective lens comes off the optical axis of the objective lens. Coma generally occurs for a light beam from an off-axis object point like this. Since coma adversely affects the recording/reproduction performance, it is necessary to remove coma as much as possible. In a conventional optical disk system including a single layer or several layers, coma occurring during lens shift is removed by designing an objective lens so as to satisfy the sine condition.
One method of increasing the capacity of an optical disk system is to increase the number of layers. This is so because a large volume of information can be held on a single disk when many recording/reproduction layers are formed on the disk. A super multilayer optical disk system like this has technical problems that are not posed in the conventional optical disk system including a single layer or several layers. One problem is an objective lens shift characteristic.
To implement a super multilayer optical disk system, the ability to suppress coma occurring when an objective lens is shifted is desirable.